Erosion-resistant surfaces are increasingly required for specific functions of corresponding products. Examples here include surfaces which in terms of the ambient conditions are exposed to high speeds in conjunction with exposure to erosive substances, such as solids or liquids. Erosive exposure is experienced in principle by objects which, firstly, themselves are moved, such as rotor blades (of wind energy systems or helicopters or boat screws), air and land vehicles (such as aircraft, rail vehicles, automobiles), and boats, for example, and, second, by objects around which or through which there is movement, such as built structures (such as architectural facing elements, power masts or wind energy towers or broadcasting towers) or pipelines, for example.
Fundamentally, erosion can be brought about by liquid or solid substances which are present themselves or dispersely or in solution in another gaseous or liquid medium (e.g., air or water) and are moved by that medium (e.g., airborne sand, rain). When these substances impinge on articles, they exert an erosive force on them. Examples of this are the erosion due to rain or airborne sand on rotor blades or in the region of the slats on aircraft.
In general terms the possibility exists of controlling wear protection, such as the erosion resistance of coatings, through the fundamentally different measures below.
For example, it is possible to increase the film thickness of the coating. In many applications, such as in aircraft construction or rotor blade construction of wind energy systems, however, this is not desirable, for reasons of weight.
Furthermore, resins with aromatic resin constituents, such as epoxy resins, can be used in the coating materials. On account of the aromatic moieties, the resultant coatings offer high wear resistance, but a significantly restricted UV stability.
It is possible, furthermore, to use coating materials comprising resins with which, induced by light or by temperature, high crosslinking densities can be achieved. For example, UV resins (via free-radical or ionic polymerization) or certain highly reactive polyaddition resins can be used. With these classes of binder it is likewise possible to enhance the wear resistance, but there are limiting factors in the case of use on large components such as rotor blades or aircraft components. In the case of formulations comprising UV resins, for example, the selection of pigments is limited, since these pigments may have absorption maxima at curing wavelengths, and the film thicknesses are limited in dependence on the levels of pigmentation. Moreover, technical challenges arise in terms of oxygen inhibition of the UV initiators. In the case where temperature-induced coating materials are used (e.g., polyurethane-based baking varnishes), a limitation exists particularly with regard to the baking temperatures in relation to plant dimensions for large components.
The coatings presently used in rotor blade or aircraft construction do not offer erosion resistance for future challenges in—for example—rotor blade construction of wind energy systems, particularly in relation to high-wind locations (offshore) or in aircraft construction (weight reduction with equal or better performance). The task, therefore, is to meet the demand for coatings which offer significant improvement in erosion resistance and therefore minimize expensive maintenance and repair intervals.
International patent application WO 2006/055038A1 to Hontek Corporation discloses erosion-resistant polyurethane coatings which are prepared from isocyanate prepolymers in conjunction with polyaspartates. The coatings do not adequately meet the requirements relating to the duration of erosion resistance. Moreover, if humidity is too low, these materials may become problematic to cure.
Means of erosion resistance include not only erosion-resistant coatings but also films. These are, for example, polyurethane elastomer films with acrylate adhesives. The films, however, are difficult to process, especially in the case of relatively large and/or multiply curved components such as aircraft or rotor blade components of wind energy systems. Components of these kinds cannot be provided uniformly with a film. In addition, the adhesive of the films often lacks sufficient durability as compared with coatings. This may result in a decrease in erosion stability.
The problem addressed by the present invention, therefore, was that of eliminating the above-described disadvantages of the prior art. The intention was to provide compositions which, as erosion-resistant coatings, feature significantly improved erosion resistance compared with erosion-resistant coatings of the prior art.
Besides the sought-after erosion resistance of the coating materials for the applications already described above, the compositions ought to offer effective resistance to the general effects of weathering (for example, UV radiation, moisture, etc.). The coatings ought further to be resistant to operating fluids such as transmission oils. The coating materials ought also to be easy to prepare and easy to process even with large components such as rotor blades of wind energy systems or aircraft.